Auto-igniter for biomass furnace

ABSTRACT

A biomass furnace ( 12 ) includes an auto-ignition system ( 28 ) for automatically resuming combustion of a biomass fuel ( 16 ) in response to sensed conditions. The auto-ignition system ( 28 ) raises the temperature of biomass fuel ( 16 ) in the combustion chamber ( 14 ), in the absence of flame or spark, by pumping superheated air into the combustion chamber ( 14 ) causing the fuel ( 16 ) to spontaneously combust. An auxiliary heat source ( 34 ) is located in a plenum ( 30 ) leading to the combustion chamber ( 14 ). A fan ( 32 ) forcibly moves air through the plenum ( 30 ), around the auxiliary heat source ( 34 ) and then into the combustion chamber ( 14 ) to initiate combustion in the absence of flame.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 61/047,791, filed on Apr. 25, 2008 which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates an ignition system for a biomass-fueled furnace, and more particularly toward an electronically controlled igniter for a biomass furnace capable of automatically reigniting fuel in a combustion chamber on a sensed demand basis.

2. Related Art

Biomass refers to living and recently dead biological material that can be used as fuel. Most commonly, biomass includes plant matter grown for use as biofuel, as well as biodegradable wastes that can be burned as fuel. Production of biomass is a growing industry as interest in sustainable fuel sources grows.

A biomass furnace is an appliance that burns biomass materials, usually in pelletized form, to create a source of distributable energy for residential, commercial and/or industrial spaces. Water and/or air is often used as an intermediate thermal fluid which is brought near to the hot combustion gases, absorbing heat energy and distributing that heat energy to a remote space to be heated through pipes, ducts or the like. By way of definition, the term “furnace” is used here in its broadest sense to mean any boiler, forced air, free-standing stove or other type of heating unit.

By slowly feeding fuel into a combustion chamber area, sometimes referred to as a burn pot, biomass furnaces can maintain continuous combustion that require little to no physical adjustments, thus creating a reliable, even and continuous source of heat energy. Biomass furnaces have become a viable, economical and popular option for heating systems.

Many of the commercially available solid biomass combustion devices, including hot water boilers, furnaces, pellet stoves and the like, rely on a high degree of user intervention to maintain consistent operation. In the case of corn stoves for example, residual klinkers must be removed from the burn pot on a regular basis. Likewise, other ash and residues must be frequently cleaned. The automation level in such stoves is very low and does not support unattended operation for extended periods. In addition, ignition of the fuel in a biomass furnace is a somewhat complicated matter. The user is typically required to manually initiate the combustion process using an extraneous ignition source, flammable liquids, gels or tinder. Ignition of the biomass fuel is so inconvenient that biomass furnaces are usually set to operate constantly at a minimum level even when there is little or no demand for heat. This continuous operation of the furnace is meant to ensure that the fire does have to be relit. Thus, at times of low demand, prior art biomass furnaces would rather waste fuel needlessly rather than inconvenience the operator to restart the combustion process.

Accordingly, there is a need for a simple ignition system for biomass furnaces which does not require any user intervention, and that can rival the customary automation levels associated with oil and gas-fired furnaces and boilers.

SUMMARY OF THE INVENTION

According to a first aspect of this invention, an automatic ignition system is provided for a biomass-fueled furnace. The ignition system comprises a plenum and a fan operatively associated with the plenum. The fan forcibly moves air through the plenum toward a biomass fuel combustion chamber. An auxiliary heat source is configured to heat the air moved through the plenum to an elevated auto-ignition temperature. At least one temperature sensor is provided, along with a control module. The control module is configured to automatically activate the auxiliary heat source in response to the temperature at the temperature sensor falling below a preset limit. According to this aspect of the invention, the automatic ignition system functions to initiate the combustion process by superheating air delivered to the combustion chamber so that biomass fuel in the combustion chamber spontaneously ignites.

According to another aspect of this invention, a biomass furnace is provided of the type for heating an intermediate thermal fluid in response to the combustion of a biomass fuel. The furnace comprises a combustion chamber that is configured to receive biomass fuel in incremental quantities and to combust the biomass fuel therein to produce hot combustion gases. An exhaust flue is provided for conducting the hot combustion gases away from the combustion chamber. A heat exchanger, proximate the combustion chamber and/or the flue, channels a thermal fluid (e.g., air or water) to absorb heat energy from the combustion gases. A plenum leads to the combustion chamber. A fan, operatively associated with the plenum, forcibly moves air through the plenum toward the biomass fuel contained in the combustion chamber. An auxiliary heat source heats the air moved through the plenum to an elevated auto-ignition temperature. At least one temperature sensor is provided along with the control module. The control module automatically activates the auxiliary heat source in response to the temperature at the temperature sensor falling below a preset limit. Accordingly, the biomass furnace is useful in both hot water and forced air type heating systems, and enables optimal cycling on and off of the combustion process on a demand basis. When the temperature at some sensed location falls below a preset limit, the control module automatically activates the auxiliary heat source, which pumps superheated air into the biomass fuel in the combustion chamber, thereby auto-igniting the biomass fuel and restarting the combustion process.

According to yet another aspect of this invention, a method is provided for igniting a solid biomass fuel in a furnace of the type for heating an intermediate thermal fluid in response to an external signal or command. The method comprises the steps of: providing a combustion chamber, holding a quantity of solid biomass fuel in the combustion chamber in the absence of flame, the biomass fuel having a characteristic auto-ignition temperature, superheating air to a temperature above the auto-ignition temperature of the biomass fuel in the combustion chamber, and injecting the heated air into the combustion chamber so that at least a portion of the solid biomass fuel in the combustion chamber spontaneously combusts.

According to yet another aspect of this invention, a method is provided for controlling the ignition of a solid biomass fuel in a biomass furnace. The method comprises the steps of providing a furnace having a combustion chamber configured to receive biomass fuel in incremental quantities, where the solid biomass fuel has a characteristic auto-ignition temperature. A quantity of the solid biomass fuel in the combustion chamber is combusted to produce hot combustion gases. A thermal fluid is channeled through the hot combustion gases to absorb heat energy therefrom and transport the heat energy to a remote space to be heated. The temperature of the thermal fluid or the space to be heated is monitored to determine a control temperature. When the control temperature reaches a predefined upper limit, the combustion step is terminated thus stopping the generation of new heat energy. When the control temperature reaches a defined lower limit, the combusting step is automatically resumed. This step of automatically resuming combustion includes raising the temperature of the solid biomass fuel above its auto-ignition temperature in the absence of flame.

Accordingly, the subject invention overcomes the shortcomings and disadvantages characteristic of prior art designs by providing a simple ignition system which does not require user intervention and that can rival the customary automation levels associated with prior art oil and gas-fired furnaces and boilers.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein:

FIG. 1 is a simplified perspective view showing a biomass fueled furnace according to the subject invention which, when fed with biomass fuel, heats water and distributes that heated water through a radiator network to warm remote spaces;

FIG. 2 is a simplified schematic view of the subject ignition system illustrating an auxiliary heat source placed in a plenum leading to the combustion chamber, together with a thermal fluid sensor and a combustion temperature sensor providing feedback information to a control module which then switches on and off the auxiliary heating unit and a fan at the appropriate times;

FIG. 3 is a schematic view as in FIG. 2, but showing an alternative forced air application where the intermediate thermal fluid is air conducted through an air duct;

FIG. 4 is an exemplary time chart depicting a control strategy for starting and stopping biomass fuel combustion in response to sensed demands as applied to a boiler scenario; and

FIG. 5 is an enlarged view of the chart region identified at 4 in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, FIG. 1 schematically illustrates a heating system, generally shown at 10. The heating system 10 includes a biomass furnace, generally indicated at 12, which in this instance is of the boiler variety. However, it will be understood that the biomass furnace 12 could be configured to run as a forced air type heating unit, or a stand-alone stove, or other type of arrangement. The biomass furnace 12 includes a combustion chamber 14, which is perhaps best shown in FIGS. 2 and 3. The combustion chamber 14, sometimes referred to as a burn pot, receives incremental quantities of biomass fuel 16 which, in FIG. 1, are illustrated in bushel sacks. The biomass fuel can be any of the known varieties, including pelletized wood, corn, soybeans, cherry pits, switchgrass, etc. The furnace 14 will include some type of hopper 18, shown here as a distended unit, into which the fuel 16 is loaded in bulk form. Fuel 16 in the hopper 18 is fed through some type of chute mechanism 20 to the combustion chamber 14 at controlled intervals. Of course, the hopper 18 and chute 20 can be integrated into the biomass furnace 12 rather than being external components.

Combustion of the fuel 16 in the combustion chamber 14 produces hot combustion gases. These hot combustion gases interact with an intermediate thermal fluid through some type of heat exchanger 21. In the case of a boiler arrangement like that shown in FIGS. 1 and 2, the heat exchanger 21 is part of a closed hot water circulation system. The water in the heat exchanger absorbs heat energy from the hot combustion gases and then transports that heat energy to a remote space to be heated, such as through one or more radiators 22. A pump 23 is shown in FIG. 2 for circulating the water in the pipes, heat exchanger and radiators. FIG. 1 illustrates an optional external tank 24 for containing the heat exchanger 21 which may or may not be used in the heating system 10 depending upon its size and configuration. Again, the illustrations here are in the nature of examples rather than specific arrangements and designs for heating systems. Combustion gases are exhausted from the furnace 12 through a flue 26.

Turning now to FIG. 2, an enlarged, simplified view of the subject automatic ignition system is shown generally at 28. The auto-ignition system 28 is intended to function with all types of biomass combustion devices including boiler types such as that depicted in FIGS. 1 and 2, forced air types such as that shown in FIG. 3, and free-standing stove configurations as well. The auto-ignition system 28 can be factory-integrated or installed as a retro-fit upgrade to an existing biomass furnace 12.

In the preferred embodiment of this invention, the auto-ignition system 28 takes advantage of the fact that nearly all solid biomass materials have an auto-ignition temperature which is defined as the lowest temperature at which the biomass material will spontaneously ignite in a normal atmosphere without an external source of ignition. In other words, without a flame or spark, a biomass material will spontaneously combust in a normal atmosphere at its auto-ignition temperature. Common solid biomass auto-ignition temperatures are in the range of 350-800° Fahrenheit.

The auto-ignition system 28 includes a plenum 30 leading to the combustion chamber 14. The plenum 30 is here shown as a tubular duct, but could be rectangular in cross-section or otherwise shaped. A fan 32 is motor driven to pump air. The fan 32 is operatively associated with the plenum 30 so as to forcibly move air through the plenum 30 toward the biomass fuel 16 contained in the combustion chamber 14. The plenum 30 and fan 32 may be either an integral part of the biomass furnace combustion process, continuously delivering combustion air to the combustion chamber 14 during normal operation of the stove, or may be a separate device altogether which is activated only during ignition and then switched off. In this latter case, the plenum 30 could be wholly separate and distinct from the delivery of combustion air to the chamber 14, or may be shared between the combustion air delivery system and the auto-ignition system 28. In other words, the auto-ignition system 28 can be either a wholly distinct unit from the normal combustion air pump system of a biomass furnace, or can be shared with the combustion air system, or can be fully integrated into the combustion air system.

An auxiliary heat source 34 heats the air moved through the plenum 30 to an elevated auto-ignition temperature. In other words, the auxiliary heat source 34 is capable of raising the temperature of air moved through the plenum 30 above the auto-ignition temperature of the biomass fuel 16 contained in the combustion chamber 14. In one exemplary embodiment of this invention, the auxiliary heat source 34 is contained within the plenum 30 in an in-line fashion like that shown in FIGS. 2 and 3. However, other configurations are possible. For example, the auxiliary heat source 34 may be located in a spur duct which communicates with the plenum 30 so as to deliver superheated air to the combustion chamber 14. Preferably, but by no means necessarily, the auxiliary heat source 34 is formed by an electric resistance heating element coiled or finned or alternatively configured to maximize heat transfer with air. The auxiliary heat source 34, when formed in the shape of a coiled resistance wire, may be similar in some respects to what might be found in an industrial heat gun or perhaps a household hair dryer. Along these lines, the Applicant's provisional application Ser. No. 61/047,791, which forms the priority document to this case, depicts various additional examples of coiled resistance wire type heating assemblies, the entire disclosure of which is incorporated here by reference. Many possible configurations of the auxiliary heating source 34 are certainly possible, and are not limited to resistance wire versions. Indeed, any form of produced heat that can be transferred or otherwise imparted to a moving air current may be implemented with effectiveness in this invention.

A control module 36 is operatively connected to the fan 32 and the auxiliary heat source 34, such as via electrical wires. The control module 36 is preferably a microprocessor based system, but may be of any type which is effective to control operation of the auxiliary heat source 34, and preferably also the fan 32, in response to sensed conditions. Along these lines, the auto-ignition system 28 includes at least one, but preferably two, temperature sensors—a thermal fluid sensor 38 and a combustion temperature sensor 40. The thermal fluid sensor 38 is positioned either in the stream of thermal fluid (FIG. 3) or in the remote space to be heated (FIG. 4) as a means of monitoring the temperature and determining a control temperature. When the control temperature reaches a predefined upper limit, the control module 36 will terminate the combustion process by any effective means. This may include discontinuing the feed of new fuel 16 into the combustion chamber 14, cutting off the supply of combustion air to the combustion chamber 14, smothering the flame in the combustion chamber 14, or by any other acceptable method. Conversely, when the control temperature reaches a predefined lower limit, the control module 36 is effective to automatically resume combustion. This step of resuming combustion includes activation of the auxiliary heat source 34, and if necessary the fan 32, to blow superheated air into the combustion chamber 14 so as to auto-ignite the fuel 16 contained therein.

It is only necessary to operate the auxiliary heat source 34 of the auto-ignition system 38 so long as fuel 16 in the combustion chamber 14 is not capable of sustained combustion. Once a sufficient flame has been established in the combustion chamber 14, the auxiliary heat source 34 can be deactivated. If the fan 32 is not required for providing combustion air, the fan 32 can also be stopped at this time. For these purposes, the combustion temperature sensor 40 is provided near the combustion chamber 14 or perhaps in the flue 26 or other suitable location, to monitor combustion gas temperatures. When the combustion gas temperatures at the point of the combustion temperature sensor 40 reach a preset limit, the control module 36 will deactivate the auxiliary heat source 34, and possibly also the fan 32, thereby discontinuing the auto-ignition operation. In this manner, the auto-ignition system 28 operates on a demand basis, pumping superheated air into the combustion chamber 14 only when it is necessary to restart the combustion process.

FIG. 3 is a view of the auto-ignition system 28 as applied in the context of a biomass furnace 12′ which operates on a forced air principle with air being moved through a duct system 42′ and using air as the intermediate thermal fluid. In this case, the heat exchanger 21′ transfers hot combustion gas energy to the air moved through the duct system 42′.

FIGS. 4 and 5 illustrate a typical control strategy for a heating system 10 according to the subject invention as applied to a boiler type configuration. Those of skill in this field will readily appreciate the adaptability of this control strategy to a forced air furnace system or a stand-alone stove type biomass furnace. This graph depicts, over a period of time, operation of a biomass furnace 12 wherein the combustion process is started and stopped depending upon the temperature sensed by the thermal fluid sensor 38. The thermal fluid sensor measurements, which define a control temperature, are represented by line 138. In this example, the predefined upper limit of the control temperature 138 has been set at about 205° F. A predefined lower limit of the control temperature 138 has been set at about 180° F. When the control temperature 138 reaches the predefined upper limit, the control module 36 intervenes to end the combustion of fuel 16 in the combustion chamber 14 using one of the techniques described above. At this point, the temperature of the thermal fluid (water in this example) steadily decreases as heat energy is continually transported to the remote space via radiators 22. After the passage of some time, the control temperature 138 reaches the predefined lower limit, which prompts the control module 36 to activate the auto-ignition system 28, whose ON/OFF status is represented by line 128 in FIGS. 4 and 5. Activation of the auto-ignition system 28 includes energizing the auxiliary heat source 34, and possibly also the fan 32 if not already running, so as to pump superheated air into the combustion chamber 14. Depending upon the system configuration, the control module 36 may also call for additional fuel 16 to be added to the combustion chamber 14 at this time.

Temperature measurements from the combustion temperature sensor 40 are depicted by line 140 in FIGS. 4 and 5. This sensor 40 monitors the temperature of combustion gases produced in the combustion chamber 14. Once the temperature sensed by sensor 40 reaches a predefined set point, indicated at 240 in FIG. 5, the auto-ignition system 28 is deactivated under the presumption that the fuel 16 in the combustion chamber 14 is capable of sustained combustion under normal operating principles. As shown in the larger graph of FIG. 4, the combustion sensor temperature 140 will continue to climb to some steady maximum operating temperature until the combustion process is halted because the control temperature 138 has reached its preset upper limit as described above.

The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention. Accordingly the scope of legal protection afforded this invention can only be determined by studying the following claims. 

1. An automatic ignition system for a biomass-fueled furnace, said system comprising: a plenum; a fan operatively associated with said plenum for forcibly moving air through said plenum toward a biomass fuel combustion chamber; an auxiliary heat source configured to heat the air moved through said plenum to an elevated auto-ignition temperature; at least one temperature sensor; and a control module operatively connected to said fan, temperature sensor and auxiliary heat source, said control module configured to automatically activate said auxiliary heat source in response to the temperature at said temperature sensor falling below a preset limit.
 2. The ignition system of claim 1 wherein said auxiliary heat source includes a heating element disposed within said plenum.
 3. The ignition system of claim 1 wherein said auxiliary heat source includes an electric resistance heating element disposed within said plenum.
 4. The ignition system of claim 1 wherein said at least one temperature sensor comprises a thermal fluid sensor, and further including a combustion temperature sensor associated with the biomass fuel combustion chamber and said control module whereby said control module automatically de-activates said auxiliary heat source in response to the temperature at said combustion temperature sensor rising above a preset limit.
 5. A biomass furnace of the type for heating a thermal fluid in response to the combustion of a biomass fuel, said furnace comprising: a combustion chamber configured to receive biomass fuel in incremental quantities and combust the biomass fuel therein to produce hot combustion gases; an exhaust flue for conducting the hot combustion gases away from said combustion chamber; a heat exchanger proximate at least one of said combustion chamber and said flue for channeling a thermal fluid to absorb heat energy from the combustion gases; a plenum leading to said combustion chamber; a fan operatively associated with said plenum for forcibly moving air through said plenum toward the biomass fuel contained in said combustion chamber; an auxiliary heat source configured to heat the air moved through said plenum to an elevated auto-ignition temperature; at least one temperature sensor; and a control module operatively connected to said fan, temperature sensor and auxiliary heat source, said control module configured to automatically activate said auxiliary heat source in response to the temperature at said temperature sensor falling below a preset limit.
 6. The furnace of claim 5 wherein said auxiliary heat source includes a heating element disposed within said plenum.
 7. The furnace of claim 5 wherein said auxiliary heat source includes an electric resistance heating element disposed within said plenum.
 8. The furnace of claim 5 wherein said at least one temperature sensor comprises a thermal fluid sensor, and further including a combustion temperature sensor associated with said combustion chamber and said control module whereby said control module automatically de-activates said auxiliary heat source in response to the temperature at said combustion temperature sensor rising above a preset limit.
 9. The furnace of claim 5 wherein said heat exchanger comprises a water transmitting pipe.
 10. The furnace of claim 5 wherein said heat exchanger comprises an air transmitting duct.
 11. A method for igniting a solid biomass fuel in a furnace of the type for heating an intermediate thermal fluid in response to combustion of the biomass fuel, said method comprising the steps of: providing a combustion chamber; holding a quantity of solid biomass fuel in the combustion chamber in the absence of flame, the solid biomass fuel having a characteristic auto-ignition temperature; heating air to a temperature above the auto-ignition temperature of the biomass fuel in the combustion chamber; and injecting the heated air into the combustion chamber so that at least a portion of the solid biomass fuel in the combustion chamber spontaneously combusts.
 12. The method of claim 11, wherein said step of heating air includes passing air through an electric resistance heating element.
 13. The method of claim 11, wherein said step of injecting the heated includes conducting air through a plenum to the combustion chamber.
 14. A method for controlling the ignition of a solid biomass fuel in a biomass furnace, said method comprising the steps of: providing a furnace having a combustion chamber configured to receive biomass fuel in incremental quantities, the solid biomass fuel having a characteristic auto-ignition temperature; combusting a quantity of the solid biomass fuel in the combustion chamber to produce hot combustion gases; channeling a thermal fluid through the hot combustion gases to absorb heat energy therefrom and transport the heat energy to a remote space to be heated; monitoring the temperature of the thermal fluid or the space to be heated to determine a control temperature; terminating said combusting step when the control temperature reaches a predefined upper limit; automatically resuming said combusting step when the control temperature reaches a predefined lower limit; said automatically resuming step including raising the temperature of the solid biomass fuel above its auto-ignition temperature in the absence of flame.
 15. The method of claim 14, wherein said terminating step includes measuring the temperature of combustion gases at a point downstream of the combustion chamber and automatically initiating said terminating step in response to the combustion gas temperature reaching a pre-set limit.
 16. The method of claim 14, wherein said step of raising the temperature includes heating air to a temperature above the auto-ignition temperature of the biomass fuel and injecting the heated air into the combustion chamber so that at least a portion of the solid biomass fuel in the combustion chamber spontaneously combusts.
 17. The method of claim 16, wherein said step of heating air includes passing air around an electric resistance heating element.
 18. The method of claim 17 wherein said step of injecting heated air into the combustion chamber includes routing air through a plenum surrounding the heating element.
 19. The method of claim 14 wherein said step of channeling a thermal fluid includes transmitting liquid water through a pipe.
 20. The method of claim 14 wherein said step of channeling a thermal fluid includes transmitting air through a duct. 